1. Field of the Invention
This invention relates to a digital switch and device interconnection system for use in communications. More particularly this invention relates to a communications system capable of integrating voice and data communications in a single network system.
2. Prior Art
A conventional business telephone switching system such as shown in FIG. 1a uses individual analog lines such as line A3-k, to connect each telephone, such as telephone A2-k, to a centrally located, expensive digital or analog switch A1, where k is a selected integer which can vary from 1 to K. Thus to make a telephone call, the analog signal from a given telephone A2-k is transmitted over one or more wire pairs A3-k to the central switch A1 and there routed to the receiving telephone, such as telephone A2-1. If central switch A1 is digital, it digitizes the incoming analog signal by means of a device known as a "codec" and transmits a digital signal, either to trunk interface A1.1 for transmission to another switch (if the addressed telephone is not handled by switch A1), or else to another part of switch A1 which converts the outgoing signal back to analog form via a codec and sends the analog signal to the addressed telephone via one or more wire pairs such as A3-1. In the conventional switching system such as shown in FIG. 1a, an expensive centralized switch A1 is required for connecting any telephone to any other telephone which it wants to call. This switch has a separate input port for connecting to the switch each telephone via an analog connection. Within switch A1 are pathways to connect each telephone port to every other telephone port. The type of pathway used depends upon the type of switch. For example, if switch A1 is a modern digital switch, the analog inputs from the telephones are digitized after they enter the switch and a technique called "time division multiplexing" is used in defining the pathways which connect one telephone to another. Switch A1 contains a complex matrix structure of a known design to allow each telephone to call any other telephone. If switch A1 is a modern "stored program controlled" switch, a sophisticated computer within switch A1 handles all calls. This computer requires extensive and complex software to perform its functions such as monitoring for off-hook and signalling information from its various telephones, and allocation of dial-tone, busy-tone, and ringing, etc. to its telephones. The operation of a stored program controlled digital switch of the type described above is well known and will not further be described here. However, the important features of a conventional stored program controlled digital switch are:
1. The telephones connect to the switch via separate analog channels. PA1 2. The signals on the analog channels are converted to digital signals inside the switch. PA1 3. The switch is complex and therefore expensive. PA1 4. The computer that controls the switch must perform a complex series of tasks for all of the telephones and therefore requires expensive and extensive software. PA1 5. Elaborate wiring is required to connect a multitude of telephones to a complex and expensive centralized switching structure. PA1 6. This switching system does not efficiently support digital data communications. PA1 1. The interfacing of telephones to the network digitally via interface devices for parallel access communications (PAC) referred to hereinafter as "PAC interface blocks" (PIBs) incorporated into the telephones, and high speed circuit switched digital connections between all of these PAC interface blocks (using an industry standard digital transmission format in the preferred embodiment). PA1 2. The digitizing of voice signals within each telephone. PA1 3. The inclusion within each PIB of a microcomputer of a type well known in the art (single or multi-chip microcomputer or similar structure) together with a relatively inexpensive logic interface circuit for handling all calls involving the particular PIB in which these components are installed. PA1 4. Relatively simple software for each microcomputer which need cover only the requirements of the single host PIB. PA1 5. A small and relatively inexpensive central equipment shelf which connects to each PIB via a multiplexed bus and simple wire interface. PA1 6. The integration of voice communications and high speed data communications in a single low cost network, with both voice and data devices interfacing to the network via the same parallel access structure.
Conventional voice communications switching systems such as illustrated in FIG. 1a are not optimal for digital data communications. Digital data communications devices such as CRT terminals can be used with such a system by converting their digital signals to analog format via a "modem" and by transmitting these analog signals to the switch via dedicated wire pairs. However, modems are expensive and are primarily useful for low speed data communications at 9600 bits per second or less. It is inefficient to convert a digital data signal to analog format for transmission, but data communications using a conventional switching system which interconnects telephones must operate within the constraints of that system. An alternative is to use a separate data communications local networking system such as the Ethernet system of Xerox Corporation or the Z-net system of Zilog, Inc. for data communications. But although these new data networking systems enable distributed data communications they have serious limitations. One primary limitation is the fact that these systems cannot be used effectively for real time interactive voice communications.
The general structure of current state-of-the-art local data networking systems such as the Ethernet and Z-net systems is illustrated in FIG. 1b by local data network B.0..
Coaxial cable B2 provides a single channel pathway for interconnection of a number of interface transceivers, B1-1 through B1-n. Each interface transceiver connects to a data station such as B3-1. The various data stations in the network communicate with each other digitally using a technique known as "packet switching" in a broadcast mode, according to the following simplified procedure:
To contact a data station B3-1 another data station B3-n will read from the communications channel constituted by coaxial cable B2 via its interface transceiver B1-n, to see if the channel is idle; i.e., to see if any other data station is transmitting on the channel. If the channel is idle, data station B3-n transmits a block of data bits called a packet onto the channel. This packet can then be read by all of the other stations which interface to coaxial cable B2. The packet includes source and destination address fields which specify the calling and called data stations. The packet also includes a stream of data bits intended for the destination data station; for this example it is data station B3-1.
While it broadcasts a packet into the communication channel of cable B2, data station B3-n reads from the channel via its interface transceiver B1-n. If what it reads is different from what it is transmitting due to contention on the channel caused by another data station transmitting at the same time, data station B3-n must wait for a period of time before retransmitting. If, on the other hand, there is no contention, data station B3-1 which is monitoring the channel will receive the packet and see that its number is in the destination address field. Data station B3-1 then assumes that the stream of data bits in the packet is intended for it.
The prior art local data networking system of FIG. 1b uses parallel connection of packet switched data stations to a common bus. A broadcast packet switched network as described above, although very useful in some strictly local applications, has a number of serious limitations. Foremost among these for the present discussion is the fact that a contention mode packet switched network as described above does not support real time interactive voice communications effectively.
Voice communications switches use a technique known as "circuit switching" in which once a pathway is established through the switch to link one telephone to another, this pathway is maintained for as long as the two users wish to talk. The pathway is dedicated to these two telephones for the duration of the call. This circuit switched pathway may be a metallic connection between the two phones or it may include a time division multiplexed channel as is done in modern digital switches.
In the distributed data networking system illustrated in FIG. 1b, information is exchanged in packets and dedicated pathways are not established between devices. Such packet switched systems do not handle voice effectively and are therefore restricted to data communications.
Up to the present time circuit switched systems have depended on a complex centralized switching structure with the many disadvantages discussed previously.
A serial "ring" distribution method as illustrated in FIG. 1c has been considered in the prior art for integrating a new key telephone set into a centralized switching system. In this method a number of telephones C11, C12, . . . , C1n are connected in series with a pair of wires used to connect each telephone to the next telephone in the ring; e.g., wire pair C21 connects telephone C11 to telephone C12. A circulating high speed digital signal is sent to each telephone via wire pairs C20, C21, . . . , C2n. The digital signal is divided into time slots with one time slot assigned to each telephone, and one time slot used for control signals between the telephones and centralized switch C3. Each telephone receives the digital signal on the wire pair which connects it to the preceding telephone in the ring, extracts the bits in its time slot, inserts any new bits it wishes to transmit, regenerates the digital signal, and outputs the signal to the wire pair which connects it to the next telephone in the ring.
A serial ring architecture as described above has serious deficiencies. Among these is the fact that failure of one telephone in the ring will result in failure of the entire ring. Also, when a telephone has failed and disrupted the entire ring, it is difficult to determine which telephone has failed since communication is no longer possible through the ring. Additionally, the number of telephones that can be supported is restricted by the number of time slots available.
A communications system which eliminates the above and other disadvantages of these prior art communications systems is required.